The Mishna (Sanhedrin 37a) teaches that "Man was created by himself (before the creation of Chava) to teach that whoever saves one life is considered to have saved the entire world... and each person must say to themselves "the world was created for me".
The Gemara in Berachos (58a) says that a good guest says "My host worked so hard to prepare good food and more, and he did it all for my sake."
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Hashem created a world that is truly enjoyable - we call it Olam Haboh.
For us to earn it, he created an "entry exam" - we call it Olam Hazeh.
As much as its thrilling to take a test in which you know the answers, it doesn't make sense to take a ten hour test - eventually you finish the exam and get your degree.
Similarly, a person can enjoy and make the most out of this world, but there reaches a point at which it is time to finish - the person dies, their actions get graded, and they get their just rewards.
Unlike other tests though, this test - their stay in this world - is fully customised to match their neshama.
One neshama may be born in the middle of the dark ages - and its test will be how it stands up to the pogroms. Another may be born in the times of the Beis Hamikdosh and its big challenge is to become a prophet and to speak to G-d. And a third may be born in the year 2200 to a world in which everything is automated, and its greatest challenge is whether or not it can lead a married life.
Furthermore, not only is the overall setting orchestrated, so is every detail during one's life - the family it is born into, the teachers and friends it lives with, the people and places who cause it pain. And these settings are constantly being adjusted - as they pass or fail each test, the next moment must bring them a test appropriate to their new standing - as no one is ever given a test that they cannot pass.
This is neccesary since the whole reason that this world was created was to give the neshama a setting in which to earn Olam Haboh, so it must have ample chance to actually earn it.
Imagine that Mike misses his bus because he was helping his wife, and because of that dashes into the office just in time to collide with a co-worker. His wife was tested whether or not to delay him, he was marked on whether or not he got upset about it, and his co-worker was graded on his reaction to getting his head bumped -
Imagine the wisdom G-d uses to make every single person go through life getting exactly the world of tests it needs, while interacting with each other!
Hence the earlier two statements in the Gemara:
1) Each person must say to themselves "the world was created for me" - As far as you are concerned the whole world was created for you. Since every test you get is part of your custom test. Even though it is true that the same thing applies for your brother in israel.
2) A good guest says "My host worked so hard to prepare good food and more, and did it all for my sake." Although Hashem had to make the world for any Neshama, we don't say "he had to make the world anyways for everyone else's sakes". He made the world for your Neshama, and he customised it in such a way that your neshama can get its reward earned - and he would have done it just for you or me.
